Make ECDSA_size() use consistent asn1 encoder.

Make ECDSA_size() use consistent asn1 encoder.

ECDSA signature lengths are calculated using i2d_ECDSA_SIG().
i2d_ECDSA_SIG() was changed in a previous PR to use a custom ASN1 encoder (using WPACKET)
so that the normal ASN1 encoder does not need to be pulled into the provider boundary.
For consistency ECDSA_size() has been changed to also use i2d_ECDSA_SIG() - this can now
be used directly inside the FIPS provider.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/10577)